The present invention relates to curable coating compositions, more particularly to coating compositions which are curable at low or ambient temperatures.
In the area of automotive refinish, repairing a defect in the original coating can involve sanding or grinding out the defect by mechanical means, or completely stripping the whole part or the whole vehicle of its original coating layers. This sanding or grinding process typically results in the exposure of bare metal which is then coated with a coating composition, usually a primer composition.
The automotive industry has, from the mid 1980's, been producing automobiles in which major automotive body parts are made of cold-rolled steel and/or steel which has an outer surface treated with a zinc-containing material. Typically the zinc coated steel used is galvanized or Galvanealed steel. Adhesion of coating compositions to these zinc coated steel substrates is problematic. Also, modification of the coating compositions to improve adhesion to galvanized substrates often impairs adhesion to cold-rolled steel substrates. Chrome-containing wash primers are sometimes used prior to the primer application in an attempt to improve adhesion to these substrates. This incorporates an added step in the refinish painting process and utilizes chrome-containing coatings which are not environmentally desirable.
Thus, it is desirable to provide coating compositions which exhibit enhanced adhesion to cold rolled steel as well as to galvanized and Galvanealed steel substrates, without the use of chrome-containing wash primers.
U.S. patent application Ser. No. 08/720,937, filed Oct. 4, 1996, abandoned, discloses a curable coating composition that has enhanced adhesion to galvanized and Galvanealed steel substrates without the use of a chrome-containing wash primer. The coating composition comprises a film-forming polymer which is the reaction product of (a) a polymer or compound containing pendant and/or terminal hydroxyl or epoxy functional groups and (b) citric acid or citric acid anhydride. These coating compositions can further comprise a curing agent selected from the group consisting of polyisocyanates and aminoplasts. For automotive refinish applications which are usually cured at ambient temperatures, the polyisocyanate curing agents are typically employed.
One disadvantage of these coating compositions which contain a polyisocyanate curing agent is the excessive length of time required for the applied coating to become tack-free to the touch (i.e., "tack-free time"), at which time the primer coating may be sanded. The ability to sand a primer coating as soon as possible after application is desirable for automotive refinish applications due to efficiency and productivity concerns.
It has been found that the use of an anhydride functional curing agent greatly accelerates the tack-free time of the above-described coating compositions while maintaining the same excellent adhesion to galvanized steel substrates without the need for chrome-containing wash primers. Incorporation of a polyisocyanate in the coating composition further improves the adhesion of the coating composition to cold-rolled steel substrates.